#1787 Improve TMX validation error reporting: Show detailed parser errors in dialog instead of only in log file

[Hiroshi Miura]

Currently, when OmegaT encounters an invalid TMX file in the project's tm/ folder, it displays a generic error dialog stating the file is invalid, while the detailed parser error messages are only written to the log file. This forces users to check log files to understand what specifically is wrong with their TMX file.

Current behavior:

Generic error dialog: "Error: Some attributes have invalid values. Use a TMX validator to find and fix them. (TF_TM_LOAD_ERROR)"
Detailed parser errors only available in log file

Requested improvement:

Show detailed parser error messages directly in the error dialog
This would help users quickly identify and fix TMX file issues without needing to examine log files

This enhancement would improve user experience by providing immediate, actionable feedback about TMX validation errors.

Reproduce Procedure

1. Create or obtain an invalid TMX file (e.g., malformed XML, invalid TMX structure, encoding issues)
2. Place the invalid TMX file in the project's tm/ folder
3. Open the project in OmegaT or reload the project
4. Observe the generic error dialog displayed
5. Check the log file to see the detailed parser error messages that are not shown to the user

Expected result: Detailed parser error information should be displayed in the user-facing dialog

Actual result: Only generic "invalid file" message shown in dialog; details only in log

[Hiroshi Miura]

https://github.com/omegat-org/omegat/pull/1666